1. Field of the Invention
The present invention relates to discharge lamp ignition apparatuses and discharge lamp ignition methods for igniting discharge lamps, and more particularly relates to discharge lamp ignition apparatuses and discharge lamp ignition methods using LC resonance.
2. Description of the Background Art
In projection type image display units (for example, projectors), discharge lamps such as high pressure mercury vapor lamps or the like are used for projecting images. Discharge lamp ignition apparatuses for igniting the discharge lamps are required to ignite the discharge lamps stably.
In general, discharge lamp ignition apparatuses have functions of: generating a high voltage which starts an electric discharge between electrodes and is applied until the electric discharge shifts to a stable operation state; and supplying a discharge lamp with a stable power as desired at a low voltage after the electric discharge between the electrodes shifts to the stable operation state.
In general, high voltage generation circuits which realize the function of generating a high voltage have a serial LC resonant circuit and apply, to electrodes of a discharge lamp, a resonant voltage obtained by a resonance phenomenon in the serial LC resonant circuit. Accordingly, the discharge lamp is ignited.
In this case, because of individual differences in values of an inductor (L) and values of a capacitor (C) in the serial LC resonant circuit, it is necessary to specify and control a resonant frequency for each serial LC resonant circuit.
For example, Japanese Laid-Open Patent Publication No. 2009-217953 discloses a discharge lamp ignition apparatus for repeatedly raising and lowering, in a range that includes a resonant frequency in a serial LC resonant circuit, a frequency of an alternating-current ramp voltage which causes an electric discharge in a discharge lamp. By applying a resonant voltage generated accordingly to the discharge lamp, the discharge lamp ignition apparatus ignites the discharge lamp stably.
However, in the above-described serial LC resonant circuit, there are not only individual difference in values of an L and values of a C but also individual differences caused by such as loss due to heat generated in coil, loss due to winding, winding capacity depending on a way of winding, temperature characteristics, and the like. Furthermore, there are individual differences caused by operations of circuits such as an inverter circuit unit which drives the serial LC resonant circuit, a power supply circuit unit which supplies a power supply voltage to the inverter circuit unit, and the like. Consequently, in conventional discharge lamp ignition apparatuses, a value of a resonant voltage generated by the serial LC resonant circuit also changes greatly.
As a result, at the resonant frequency in the serial LC resonant circuit, a high voltage for stably igniting the discharge lamp may not be obtained successfully, resulting in a possibility of ignition failure of the discharge lamp.
On the contrary, there is also a case where a peak value of the resonant voltage at the resonant frequency is raised greatly, and, at the time of driving at the resonant frequency, an unexpected excessive current flows in or an unexpected excessive voltage is applied to the discharge lamp and circuit elements that constitute the discharge lamp ignition apparatus, resulting in a possibility of breakage of the discharge lamp and the circuit elements.